1. Field of the Invention
This invention relates to liquid crystal light valve (LCLV) systems and methods that incorporate birefringence phase compensation.
2. Description of the Related Art
Liquid crystal light valves useful for projecting bright images are known in the art, and are discussed for example in Bleha et al., "Application of the Liquid Crystal Light Valve to Real Time Optical Data Processing", Optical Engineering (The Journal of the Society of Photo-Optical Instrumentation Engineers), Jul./Aug. 1978, Vol. 17, No. 4, pages 371-384. The light valve described in this publication includes a twisted nematic liquid crystal that controls the transmission of light from a polarizer to an analyzer having an optical axis that is oriented perpendicular to the polarizer's optical axis. In the null state, the axis of the projection beam is rotated in one direction during the initial pass through the liquid crystal, and is then rotated in the opposite direction during the return pass so that the beam cannot be transmitted through the analyzer. In the "on" state, an applied electric field across the liquid crystal causes the liquid crystal's optical axis to be tilted toward the direction of the electrical field. This introduces a phase retardation between the polarization components of the light which are parallel and perpendicular, respectively, to the liquid crystal's optical axis. As a result, linearly polarized light incident on the liquid crystal is reflected back through the liquid crystal as elliptically polarized light which includes a component transmitted through the analyzer.
The quality of the image produced by such a light valve depends in large part upon the contrast ratio between the null state and the "on" state of the liquid crystal. Specifically, the less light that is transmitted through the liquid crystal in the null state, the greater is the contrast ratio. A problem arises, however, when more than one wavelength of light is to be controlled by the liquid crystal. For example, if the liquid crystal light valve (LCLV) is to be used in a color video system, it would be desirable to project (for example) red, blue and green light onto the liquid crystal. The incident light beam is resolved into two components in the liquid crystal that have polarization electric field vectors parallel and perpendicular, respectively, to the major optical axis of the liquid crystal whenever the polarization direction of the incident light and the liquid crystal's optical axis are not parallel. Thus, a phase delay is introduced in the liquid crystal between the two components. This phase delay arises because the liquid crystal exhibits different refractive indices to the two light components, so that the two components travel through the liquid crystal at different velocities. The light transmitted through the liquid crystal is at least slightly elliptically polarized due to the phase delay between the two components so that, even in the null state, there is a small component of light that passes through the analyzer. The birefringence of a liquid crystal is dependent on wavelength; as wavelength increases, birefringence gradually decreases in the visible region. This makes it difficult to achieve a perfect dark state during null voltage for a broadband light source, and thereby degrades the system's contrast. For example, if the null state transmission is minimized at one wavelength, it will usually cause light leakage at other wavelengths, thus making it impossible to achieve perfect darkness in the null state when a plurality of colors is used.
A phase compensation technique has previously been developed to improve the LCLV's contrast ratio. The technique involves passing light through two separate liquid crystal layers that have their major optical axes aligned perpendicular to one another at the intersection between the two liquid crystal layers. This technique has been applied to parallel-aligned light valves (Wiener-Avnear et al., U.S. Pat. No. 4,466,702, assigned to Hughes Aircraft Company); twisted nematic light valves (Wiener-Avnear; U.S. Pat. No. 4,408,839, assigned to Hughes Aircraft Company) and two supertwisted nematic liquid crystal cells (Katoh et al., "Application of Retardation Compensation; A New Highly Multiplexable Black-White Liquid Crystal Display With Two Supertwisted Nematic Layers", Japanese Journal of Applied Physics, Vol., 26, No. 11, Nov. 1987, pages L1784-L1786).
To achieve proper compensation, the compensation cell should be identical to the master cell. The contrast is sensitive to any phase mismatch between the compensation and master cells; a small thickness difference between the two from manufacturing tolerances can greatly degrade the contrast ratio. Temperature variations can produce the same undesirable effect.
The fixed phase retardation compensation approach described above also makes no improvement to the light valve's response time. This is particularly important for full-color direct view and projection displays, in connection with which nematic liquid crystals have been widely employed. For high resolution projection displays, three separate liquid crystal cells are used with each cell optimized for each color, such as red, green and blue. A major difficulty of this three-cell approach is in accurately aligning the corresponding pixel elements of the different cells. Processing all three colors with a single cell would eliminate this pixel registration problem. However, the liquid crystal response time must be at least three times faster (less than 5.5 ms) than that of the three-cell approach to achieve the required 180 Hz color-sequential operation (for a 60 Hz system). Unfortunately, the response time of a twisted nematic or supertwisted nematic liquid crystal cell is about 5 to 10 times too slow for this purpose.
High speed nematic liquid crystal modulators with response times less than 100 microseconds have been demonstrated recently with a parallel-aligned liquid crystal cell (S. T. Wu, "Nematic Liquid Crystal Modulator With Response Time Less Than 100 .mu.s at Room Temperature", Applied Physics Letters, Vol. 57, pages 986-988 (1990). However, the contrast ratio of a parallel-aligned cell is good only for a narrow band laser wavelength. As the spectral bandwidth increases, the contrast ratio decreases dramatically. This severely limits the usefulness of the new parallel-aligned cell for full color systems.